The third person singular and plural forms, as well as first person singular form, are no doubt very common as the standard voice of the narrator in written works.

But for second person singular/plural and first person plural, I would imagine it's markedly less so. Are those conjugations (and for the imparfait du subjonctif as well) as rare as I'm guessing? And does that mean many people don't even know them? I realize for the regular verbs they just follow a pattern, but for the irregular verbs, surely that must border on uncommon knowledge.

Or are the patterns sufficient to know them all just by knowing a few?

The conjugations of the first and second verb groups are hardly problematic (except for the a tendency to write -at at the 3rd person singular). A lot of people will hesitate on the correct conjugation of irregular verb, though, and even for an avid reader like me, that conjugation is passive, not active knowledge.

In general, it's so rare for people to actually need to write down a passé simple that I don't see why you should judge someone over it. Most English-speakers can't use the subjunctive properly, and I know more foreign than native speakers of English that can tell lie and lay apart (I can barely manage sometimes, myself), so French hardly has a monopoly on this sort of thing.

The passé simple isn't even necessarily the tense where the most variation is seen: I've found alternate future conjugations for asseoir, résoudre et craindre. Still it's hard to beat the three different forms found in the passé simple for the verbs derived from traire (That verb may have fallen out of use, but many people have needed a passé simple form for distraire, soustraire ou extraire!).

I don't understand this bit about traire....I looked and can't find any passe simple conjugation for it, let alone three different ones. What's going on here??
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AerovistaeJun 18 '14 at 14:48

It's easy for grammar writers to pretend that (say) extraire has no passé simple. People who actually speak and write French? They can and will conjugate it in that tense and a linguist can't afford to dismiss these as mere mistakes. Try a google book search for "il extraya" and see for yourself! This is no different from the American conjugating dive as dove in the past participle, but the brits conjugate it as dived.
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CirceusJun 18 '14 at 14:57

@Circeus Well, defectiveness and insecurity should not be ignored either. If you ask an American speaker about DIVE, his reaction will probably depend upon which form he uses first: preterit dived or past participle dove. In my experience, producing a first form is easy but the second one makes the speaker insecure. As for French, closons is also found in Google Books searches but its frequency compared to clôturons (and the opposite distribution of closent/clôturent) still leans towards defectiveness.
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GAM PUBJun 18 at 8:42

Some forms of conjugations are becoming obsolete and are more and more used in books and in elevated languages forms only. The passé simple de l'indicatif and the imparfait du subjonctif are in this case, as well as their compound forms, i.e. the passé antérieur de l'indicatif and the plus-que-parfait du subjonctif. The passé composé is replacing the passé simple at the indicative mode and the imparfait at the subjunctive mood. As for anteriority, the plus-que-parfait de l'indicatif is replacing the passé antérieur de l'indicatif. Conditional past tenses (1st and 2nd forms) are also disappearing. However some expressions containing those deprecated past tenses will remain in the language, as a relic of their bright past.

These things prove that the French idiom is not dead at all. It is still alive.